The role of Zn and Ru substitution on the structural, magnetic and dielectric properties of ferrite cuprospinels

Abstract

A comprehensive study on the structural, magnetic and dielectric behavior of ferrite cuprospinels with site specific substitution of non-magnetic Zn and weakly magnetic Ru ions has been presented. Such substitution leads to two classes of stable crystal symmetries (i) Tetragonal (I41/amd): CuFe1.95Ru0.05O4 and Cu0.95Zn0.05Fe1.95Ru0.05O4 and (ii) Cubic (Fd-3m): Cu0.60Zn0.40Fe1.95Ru0.05O4 and Cu0.05Zn0.95Fe1.95Ru0.05O4 bulk polycrystals. The former set of compounds undergo significant Jahn-Teller (JT) distortion due to high Cu content and such distortion gets lifted with the substitution of non-magnetic Zn ions. Ferrimagnetic (FiM) behavior is predominant in all these compounds except the highest Zn (95 at%) content cubic system with diluted substitution of Ru (2.5 at%) which exhibits antiferromagnetic (AFM) ordering below the Néel temperature (TN ~ 10.9 K). We employed Néel’s expression for FiM systems applicable for the two sublattice model to extract the dominant exchange interactions (JAA, JAB and JBB) between the A-site and B-site cations. For the tetragonally distorted Zn diluted system exhibits JAA/kB ~391 K with very high asymptotic Curie temperature, while the Zn-rich cubic system exhibits JAA/kB ~ 300 K. On the other hand, the tetragonally distorted spinel devoid of Zn exhibits dominant JBB/kB ~ 229 K with negligible JAA/kB and moderate JAB/kB (65 K) signifying that the JT active ions cause the onset of magnetic frustration and enhanced ordering temperatures. The ac-resistivity ρ(f,T) studies reveal that the mode of conduction follows Mott’s variable-range hopping (VRH) model with a linear logarithmic variation of ρac versus T-0.25. The temperature dependence of the VRH activation energy (24.5 meV ≤ Ea-VRH ≤ 67 meV at T = 50 K and 94 meV ≤ Ea-VRH ≤ 257 meV at T = 300 K) of charge carriers follows a linear behavior which indicates the presence of localized polaron in all the investigated systems.